The present invention relates to data communication techniques. In particular it relates to the detection of the presence or absence of a remote signal in a modem that uses echo cancellation.
In the field of data communications, it is often advantageous for data traffic to be carried over a single communications channel, e.g., a "two-wire" circuit, in both directions simultaneously between a near-end and a far-end, or remote, modem. This simultaneous, two-way, transmission is known as `full-duplex` operation. In order to accommodate full-duplex operation, signal energies from both directions of transmission must be superimposed on this single communications channel simultaneously. As a result, some signal energy from each direction of transmission inevitably leaks through to the opposite direction path. This cross-path leakage allows some of a modem's transmitted signal to appear at the input of tis own receiver. This undesired leaked-through signal is often referred to as "echo." An echo signal can seriously degrade a modem receiver's performance if not mitigated.
Although the echo cannot simply be filtered, it can be reduced to acceptable levels through the use of an adaptive "echo canceler" within the modem. The echo canceler adaptively synthesizes a replica of the echo, which is then subtracted from the received signal, leaving a substantially echo-free signal. This echo-compensated signal is applied to the modem'data receiver. The echo canceler technique has been so successful that it now is a standard component in most full-duplex high-speed modems.
However, modems which employ an echo cancellation technique have a unique difficulty in reliably detecting the presence and absence of the remote signal. Usually, in an echo-canceling modem, an energy detector is placed after the echo canceler in order to measure the combined energy of the remote signal, noise and residual echo. This type of detector provides an estimate of the remote signal plus noise only as long as the residual or uncanceled echo remains relatively small. However, when the connection to the remote modem is abruptly dropped, the resulting change in impedance of the communications channel to the near-end modem causes the residual echo to increase. As a result, the energy detector detects the energy due to the increased signal level of the residual echo and does not detect the disconnect--even through the remote signal has disappeared. When this happens, a user must manually disconnect the modem before attempting to reestablish the connection.
It should also be noted that a modem's receiver can also use its measure of the mean-squared-error (MSE) between the receiver's estimate of the received data and the receiver's equalizer output as an indicator that the connection to a remote modem has been dropped. For example, when the connection to the remote modem is dropped, the MSE increases. However, in echo-canceling modems it has been observed that the receiver, through influence of the equalizer, carrier recovery and timing recovery circuits, may still "lock-on" to the echo signal notwithstanding that during the disconnect of the remote modem the MSE briefly increased.